Daylight in Building Design Planning for Daylight at the by mikesanye


									                                                     Daylight in Building Design 2.

For centuries, daylight was the only efficient source of light available. Architecture was
dominated by the goal of spanning wide spaces and creating openings large enough to
distribute daylight to building interiors. Efficient artificial light sources and fully glazed
facades have liberated designers from these constraints of the past. Advanced daylighting
systems and control strategies are another step forward in providing daylit, user-friendly,
energy-efficient building environments. These systems need to be integrated into a
building’s overall architectural strategy and incorporated into the design process from
its earliest stages. This chapter outlines the design considerations associated with enhancing
a building’s daylight utilization while achieving maximum energy efficiency and
user acceptance.

               Planning for Daylight at the Conceptual Design Phase 2.1.

                                                                                                    Table 2-1:

                                                                                                    The process of

                                                                                                    designing for

                                                                                                    daylight in buildings

                                                                                                 daylight in building design 2-1
                                  Daylighting strategies and architectural design strategies are inseparable. Daylight not only
                                  replaces artificial lighting, reducing lighting energy use, but also influences both heating
                                  and cooling loads. Planning for daylight therefore involves integrating the perspectives and
                                  requirements of various specialities and professionals. Daylighting design starts with the
                                  selection of a building site and continues as long as the building is occupied.

                                  Daylighting planning has different objectives at each stage of building design:
                                      •    Conceptual Design: As the building scheme is being created, daylighting design
                                           influences and/or is influenced by basic decisions about the building’s shape,
                                           proportions, and apertures, as well as about the integration and the role of
                                           building systems.
                                      •    Design Phase: As the building design evolves, daylighting strategies must be
                                           developed for different parts of the building. The design of facades and interior
                                           finishing, and the selection and integration of systems and services (including
                                           artificial lighting), are all related to the building’s daylighting plan.
                                      •    Final/Construction Planning: The selection of materials and products is
                                           affected by the building’s daylighting strategy; final details of the daylighting
                                           scheme must be worked out when construction plans are created.
                                      •    Commissioning and Post-Occupancy: Once the building is constructed, lighting
                                           controls must be calibrated, and ongoing operation and maintenance of the
                                           system begins.

                                  2.1.1. Daylight Availability
                                  All daylighting strategies make use of the luminance distribution from the sun, sky,
                                  buildings, and ground. Daylight strategies depend on the availability of natural light,
                                  which is determined by the latitude of the building site and the conditions immediately
                                  surrounding the building, e.g., the presence of obstructions. Daylighting strategies are also
                                  affected by climate; thus, the identification of seasonal, prevailing climate conditions,
                                  particularly ambient temperatures and sunshine probability, is a basic step in daylight design.
                                  Studying both climate and daylight availability at a construction site is key to understanding
                                  the operating conditions of the building’s facade. The daylighting design solution for the
                                  building should address all of these operating conditions.

                                                                  There are several sources of information on daylight
                    Figure 2-1:                                   availability [Dumortier 1995]. For example, daylight
              Sunpath diagram                                     availability data has been monitored every minute at more
                projected on a                                    than 50 stations worldwide since 1991 (
            fisheye photograph                                    and has also been monitored in the Meteosat satellite every
                    of the site                                   half hour from 1996–1997 (under beta testing).

                                                                  High latitudes have distinct summer and winter conditions;
                                                                  the seasonal variation of daylight levels is less apparent at

2-2 daylight in buildings
low latitudes. At high latitudes where winter daylight levels are low, designers usually aim
to maximize daylight penetration in a building; redirection of daylight into buildings
from the brightest regions of the sky is an appropriate strategy at these latitudes. By contrast,
in the tropics where daylight levels are high throughout the year, the design emphasis is
usually on preventing overheating by restricting the amount of daylight entering the
building. The obstruction of large parts of the sky, especially of areas near the zenith, and
the admission of daylight only from lower parts of the sky or of indirect light reflected from
the ground are useful strategies in tropical regions.

Daylight availability strongly depends not only on the latitude but also on a building’s
orientation; each orientation will require a different design emphasis. Study of vernacular
architecture and past successful daylighting designs is a good way to understand the
relationship between climate and building design.

2.1.2. The Building Site and Obstructions
At a construction site, the sky is usually obstructed to some extent by surrounding
buildings and vegetation.

                                                                                                       Figure 2-2:

                                                                                                       Shading study in a

                                                                                                       large-scale urban

                                                                                                       housing area

                                                                                                       in Berlin. Existing

                                                                                                       building blocks

                                                                                                       are shown in black,

                                                                                                       proposed new

                                                                                                       buildings in grey.

                                                                                                       The study was

                                                                                                       carried out with

                                                                                                       a physical model in

                                                                                                       an artificial sky.

                                                                                                       The picture shows

Studying the obstructions at a construction site tells a designer about the daylight potential         the superposition

of the building’s facades and allows him or her to shape the building and to allocate floor            of shadows for

areas with respect to daylight availability. In many cases, buildings are self-obstructing,            08:00, 10:00, 12:00,

so building design and obstruction studies become interconnected.                                      14:00, and 16:00 on

                                                                                                       March/September 21.

Local zoning regulations limit a building’s design (e.g., building size, height, etc.) and the
impact a new building can have on surrounding, existing buildings. The latter restrictions
have their origins in fire protection, imposing minimum distances between neighbouring

                                                                                                    daylight in building design 2-3
                            buildings to prevent fire from spreading. These regulations evolved into legislation to
                            protect the right to daylight, originally drafted (as early as 1792) when powerful sources
                            of artificial light were unknown or unavailable to the majority of the population, and the
                            availability of daylight was essential in building interiors. In selecting daylighting strategies,
                            a designer must take into account the degree to which the new building will create an
                            obstruction for existing buildings, reducing their access to daylight, and/or will reflect
                            sunlight that might cause glare at the street level or increase thermal loads in
                            neighbouring buildings.

                            Zoning regulations and floor area indexes that regulate the extent of urban density also
                            affect daylighting design. The aim of maximizing floor area in order to get the best
                            economic return from a new building may conflict with the design goal of providing
                            interior daylight.

                            Several methods and tools are available to analyse obstructions. The basic approaches are:
                                •    plotting the “no-sky line” on the work plane of a selected space; the no-sky line
                                     divides points on the work plane that can and cannot “see” the sky [Littlefair 1991];
                                •    examining obstructions from one specific view point by projecting the sun’s course
                                     or a daylight availability chart on a representation of the building site
                                     (Figure 2-1);
                                •    computing the amount of incident daylight and radiation for specific locations and
                                     orientations on the site; or
                                •    projecting shadows that will fall on the facade or ground when the sun is in
                                     specific positions; this approach gives an overview of the availability of sunlight
                                     at the site (see Figure 2-2).

                            For heavily obstructed facades, daylight-redirecting systems can improve the distribution
                            of light to interior spaces. Glass prisms have been used for this purpose for more than a
                            hundred years; today a range of systems can be used, including holographic elements, laser-
                            cut panels, and anidolic elements.

                            2.1.3. Building Schemes and Building Types
                            Commonly encountered constraints on different building types over the years have
                            resulted in typical building shapes and design schemes for standard types of building uses.
                            These schemes generally incorporate daylighting strategies from which designers can learn.

                            Daylight design and building design can merge to different degrees. In some buildings,
                            such as churches (see Figure 2-3), the daylighting strategy and the building design
                            scheme are almost identical; in buildings where the organization of floor areas is
                            complex, daylight is treated as one design issue among a host of others. The more that
                            daylight is the generating factor for a design, the more the daylighting strategy is an
                            architectural strategy.

2-4 daylight in buildings
                                                                                                    Figure 2-3:

                                                                                                    Daylighting design

                                                                                                    strategies for

                                                                                                    different building

                                                                                                    types. Churches (top

                                                                                                    row): Pantheon,

                                                                                                    Roman hall church,

                                                                                                    gothic basilica,

                                                                                                    modern. Schools

                                                                                                    (middle row): linear

                                                                                                    atrium, cross of

                                                                                                    Hamburg, nucleus,

                                                                                                    courtyards. Office

                                                                                                    buildings (bottom

                                                                                                    row): cellular, open

                                                                                                    plan, group offices,

                                                                                                    combination offices

Different organisations of building floor space develop in response to different needs. The
bottom row of Figure 2-3 shows various ways of organizing space in office buildings. It
is easy to see that a cellular design and
an open plan design, for example, will
demand different daylighting strategies.                                                            Figure 2-4:

A conventional window may be                                                                        Light and shadow

adequate to distribute daylight to a                                                                in the Ronchamp

shallow office room, but bringing                                                                   chapel designed by

daylight into deep spaces requires more                                                             Le Corbusier

complex design strategies.

One of the first steps in planning for
daylight is to list all of a project’s floor spaces and determine the lighting requirements of
these areas. The required daylight level and degree of control over the visual environment
are among the most important criteria (see Chapter 3).

Performance parameters are usually objective design criteria; however, the attractiveness
of spaces cannot be expressed in purely quantitative terms. The work of architects such
as Alvar Aalto, Le Corbusier, and Louis Kahn show how to use architectural design
features to create impressive spaces with daylight (Figure 2-4).

                                                                                                 daylight in building design 2-5
                                  The building’s overall design scheme determines daylighting strategies and daylight
                                  potential in all building zones; therefore, performance parameters should be checked during
                                  the initial design phase. Incorrect assumptions about the distribution of daylight within the
                                  space will result in poor daylighting performance.

                                  During the initial design phase, the daylighting designer’s goal is to make sure that the
                                  specified performance can be achieved within the framework of the design. The proportion
                                  of spaces in relation to apertures should be checked. If the performance of the daylight
                                  strategy depends on the performance of particular daylighting systems, these systems have
                                  to be included in the prediction method. Rules of thumb, graphical methods, and
                                  simulation of daylight with physical or computer models are applicable at this stage of
                                  the design process (see Chapter 6). Most of these methods do not adequately account
                                  for a design’s thermal behaviour even though the thermal strategy and the daylighting
                                                                                   strategy   are   inseparably    linked;      a
                                                                                   daylighting design should therefore include
                    Figure 2-5:
                                                                                   thermal calculations.
               Typical building
               of the late 20

             century that does
                                                                                   2.1.4. Retrofitting/Refurbishment
                 not comply to
                                                                                   In most industrialised countries, the
            today’s daylighting
                                                                                   proportion of retrofit activities in the
                                                                                   construction sector has increased steadily
                                                                                   during the past two decades. Today, a
                                                                                   large number of buildings are refurbished
                                                                                   because of:
                                                                                     •    a poor indoor environment (air
                                                                                          quality, visual environment, etc.),
                                                                                     •    high energy consumption,
                                                                                     •    a poor state of repair, or
                                                                                     •    the need for a new floor layout.

                                                                                   Daylight design is an important component
                                                                                   of a retrofit when building components
                    Figure 2-6:
                                                                                   that affect the building’s daylighting
                Power Gen, UK.
                                                                                   performance are replaced. Common retrofit
          The daylight strategy
                                                                                   measures include replacement of windows
               and the building
                                                                                   or of the whole facade; old windows are
           design strategy are
                                                                                   often leaky and thus a source of heat loss.
             inseparably linked
                                                                                   Refurbishment is a chance not only to
                 to each other.
                                  replace old building components with new ones, but also to redefine the functional
                                  concept of a building in order to meet today’s requirements.

2-6 daylight in buildings
Selection of the right glazing is of major importance for a building’s daylighting strategy.
The combined application of new glass and new daylighting systems, particularly those
that provide solar shading, glare control, and the redirection of light, can increase daylight
and decrease cooling loads. Daylighting measures are only efficient when the performance
of artificial lighting systems is also addressed, i.e., new efficient lamps and luminaries and
an advanced control system are installed. Combining daylighting and artificial lighting
systems through, for example, a combined control strategy or the integration of lamps in
an interior light shelf, is a design option in retrofits as well as new construction.

The increasing tendency to replace heating, ventilation, and air conditioning (HVAC)
plants with hybrid HVAC-thermal-lighting systems and hybrid or natural ventilation
strategies will affect the building envelope design. HVAC plant sizing and redesign should
be integrated with envelope design because significant load reductions can occur as a result
of new window and daylighting technologies.

                                             Daylighting Strategies for Rooms 2.2.

The aims of room daylighting are to
adequately illuminate visual tasks, to                                                              Figure 2-7:
create an attractive visual environment,                                                            Window, clear view
and to save electrical energy. Both the
building design scheme and the
application of systems play roles in
meeting these goals.

The performance of a daylighting
strategy for rooms depends on:
                                                                                                    Figure 2-8:
    •    daylight availability on the
                                                                                                    Window with exterior
         building envelope which
                                                                                                    louver blinds, where
         determines the potential to
                                                                                                    the view is partially
         daylight a space;
    •    physical and geometrical
         properties of window(s), and
         how windows are used to
         exploit   and    respond    to
                                                                                                    Figure 2-9:
         available daylight;
                                                                                                    Window with interior
    •    physical and geometrical
                                                                                                    vertical lamellas,
         properties of the space.
                                                                                                    where the view is

                                                                                                    completely obstructed

                                                                                                 daylight in building design 2-7
                            2.2.1. Function of Windows
                            The old definition of a window as an aperture in an opaque envelope is no longer strictly
                            applicable. Innovations such as fully glazed skeleton structures and double-skin facades
                            defy the scope of this definition. Nevertheless, we will use the term “window” to analyse
                            daylighting strategies. Windows have several functions, which vary depending on the
                            individual design case.

                            One key function of a window is to provide a view to the outside. View plays an important
                            role in an occupant’s appraisal of the interior environment even if the exterior environment
                            is not especially attractive. The size and position of windows, window frames, and other
                            elements of the facade need to be considered carefully in relation to the eye level of building
                            occupants. Daylighting systems can affect the view to the outside. If an outdoor view is a
                            priority in a daylighting design, visual contact with the exterior has to be maintained under
                            all facade operating conditions. Advanced daylight strategies therefore often allocate
                            different functions to different areas of the facade or to different facades. View windows
                            then can be preserved without being compromised by other functions.

                            Daylighting is one of the main functions of windows. The window design determines the
                            distribution of daylight to a space. Windows chosen solely for their architectural design
                            features may perform satisfactorily in many cases. For dwellings and other buildings that
                            have relatively minimal visual requirements, application of advanced daylighting systems
                            is not usually appropriate.

                            Advanced daylighting systems can be useful in cases where:
                                •    difficult tasks are performed, and a high degree of control over the visual
                                     environment is required;
                                •    the building’s geometry is complex, e.g., there are heavily obstructed facades or
                                     deep rooms;
                                •    control of thermal loads is required (adjustable solar shading can be an effective
                                     strategy in this case).

                            Daylighting is inseparably linked to solar gain. In some design cases, added solar gains
                            from daylighting may be welcome; in other cases, heat gain must be controlled. If solar
                            gains are desirable, windows are a good way to provide them. In general, the goal of
                            building design is to reduce cooling loads. There are a number of ways to control solar
                            gains from windows and facades; the simplest method is the direct gain approach, where
                            a shading system simultaneously controls the visual and thermal environments. More
                            advanced techniques, such as collector windows and double-skin facades, allow some
                            degree of separate control over the thermal and visual environments. In passive solar
                            architectural concepts, solar gains are controlled by the orientation and the application of
                            shading systems as a function of the sun’s position.

2-8 daylight in buildings
The operability of windows needs to be considered when daylighting systems are selected.
Shading systems located in the window pane do not work properly when the window is
open; if daylight-redirecting systems are attached to the window, the window’s operation
will have an impact on the systems’ performance. Operable windows also often serve as
fire escapes. The impact of fire balconies on daylight performance needs to be considered.

Glazed areas are an interface between exterior and interior; therefore, windows involve
a number of design considerations. Aside from the above-mentioned primary functions,
the following issues are especially important for glazed areas:
    •    glare,
    •    privacy/screening of view,
    •    protection from burglary.

2.2.2. Design Strategies for Windows
A window system must address the range                                                              Figure 2-10:

of a building’s exterior conditions to                                                              Large rooflights

fulfil the range of interior requirements.                                                          without shading

The placement and sizing of windows                                                                 system in cloudy

are among the most powerful features of                                                             England

architectural design for daylight. Because
the design of windows has a decisive
effect on the potential daylight and
thermal performance of adjacent spaces, it needs to be checked very carefully [O’Connor
et al. 1997]. The LT (Light-Thermal) method, which was developed for typical climates in
the European Union, allows the estimation of energy consumption for heating, lighting,
and cooling as a function of glazing ratio [Baker and Steemers 2000]. Simple design tools
(see Chapter 6) allow a quick evaluation of window design and room geometry.

Windows are almost always exposed to the sky; daylighting systems can adapt windows
to changing sky conditions and transmit or reflect daylight as a function of incident angle.
Daylighting systems are primarily used for solar shading, protection from glare, and
redirection of daylight. Whether or not daylighting systems are required to support the
performance of window systems, and which system or systems is appropriate, are key
decisions in the design process. See Chapter 4 for a detailed description and evaluation
of innovative daylighting systems.

The adjustment of daylighting strategies to specific sources of skylight is an important
characteristic of daylighting strategies.

Strategies for Skylight
Strategies for diffuse skylight can be designed for either clear or cloudy skies; however,
the most significant characteristic of these strategies is how they deal with direct sunlight.

                                                                                                 daylight in building design 2-9
                             Solar shading always is an issue for daylighting except on north-oriented facades (in the
                             northern hemisphere). If solar shading is only of minor importance as a result of orientation
                             and obstructions, a system to protect from glare can be used for solar shading as well.

                             Solar shading and glare protection are different functions that require individual design
                             consideration. Solar shading is a thermal function that primarily protects from direct
                             sunlight, and glare protection is a visual function that moderates high luminances in the
                             visual field. Systems to protect from glare address not only direct sunlight but skylight and
                             reflected sunlight as well.

                             Strategies for Cloudy Skies
                             Daylighting strategies designed for diffuse skylight in predominantly cloudy conditions aim
                             to distribute skylight to interior spaces when the direct sun is not present. In this case,
                             windows and roof lights are designed to bring daylight into rooms under cloudy sky
                             conditions, so windows will be relatively large and located high on the walls. Under sunny
                             conditions, these large openings are a weak point, causing overheating and glare. Therefore,
                             systems that provide sun shading and glare protection are an indispensable part of this
                             strategy. Depending on the design strategy, various shading systems that transmit either
                             diffuse skylight or direct sunlight may be applicable in this case. To avoid decreasing
                             daylight levels under overcast sky conditions, moveable systems are usually applied.

                             Some innovative daylighting systems are designed to enhance daylight penetration under
                             cloudy sky conditions (see the classification of systems in Chapter 4). Some of these systems,
                             such as anidolic systems or light shelves, can control sunlight to some extent. The
                             application of simple architectural measures, such as reflective sills, is another opportunity
                             to enhance daylight penetration, but the design of the window itself is the main influence
                             on the performance of this type of strategy under cloudy conditions.

                             Strategies for Clear Skies
                             In contrast to daylighting strategies for cloudy skies, strategies that diffuse skylight in
                             climates where clear skies predominate must address direct sunlight at all times. Shading
                             of direct sunlight is therefore part of the continuous operating mode of this strategy.
                             Openings for clear sky strategies do not need to be sized for the low daylight levels of
                             overcast skies. Shading systems that allow the window to depend primarily on diffuse
                             skylight are applicable in this case (see Chapter 4).

                             Direct Sunlight
                             Strategies for sunlight and diffuse skylight are quite different. Direct sunlight is so bright
                             that the amount of incident sunlight falling on a small aperture is sufficient to provide
                             adequate daylight levels in large interior spaces. Beam daylighting strategies are applicable
                             if sunshine probability is high. Since sunlight is a parallel source, direct sunlight can be
                             easily guided and piped. Optical systems for direct light guiding and systems for light
                             transport are applicable in this case (see Chapter 4). Apertures designed for beam

2-10 daylight in buildings
daylighting do not usually provide a view to the outside
and should therefore be combined with other view                                                   Figure 2-11:
openings (¡ Palm Springs Chamber of Commerce ).                                                    Homogeneous

Because beam daylighting requires only small                                                       window design

apertures, it can be applied as an added strategy in an
approach that otherwise focuses on cloudy skies.

2.2.3. Functional Division of a Window
If a designer can allocate one predominant function
to a window, he or she can design it for optimum
performance that will not be compromised by
contradictory requirements. The designer must then
make sure that all windows together fulfil the full
range of requirements in a room.                                                                   Figure 2-12:

When a window has to satisfy several functions in any                                              window design
operation mode, the range of applicable daylighting
systems is constrained because the system selected
must take account of all of the window’s functions. The
design approach for this type of opening therefore
usually consists of applying moveable systems that
can be recessed when not needed. The designer should
consider controlling systems using a building energy
management system because they might not otherwise
be operated appropriately.

The heterogeneous design of a window allots specific functions to specific areas of a
window. Different daylighting systems can be applied to different parts of the window, or
similar systems may be operated separately for different areas of the window. The
interaction of daylighting systems in this case needs detailed design consideration.

2.2.4. Strategies for Fenestration
Whether to use sidelighting or toplighting, unilateral or multilateral daylighting strategies
should be decided during a building’s conceptual design stage.

Although unilateral sidelighting is the standard daylighting case, its implementation
requires care. It aims to distribute daylight into the depth of a space, to provide enough
light to perform a task in the room while avoiding glare and allowing a view to the outside.
Because these ambitions may conflict, the division of a facade into openings with specific
functions is a promising way to apply sidelighting (¡ Willy Brandt Building).

 ¡This notation is given for case study buildings documented in the Survey of Architectural
Solutions, which is included on the CD-ROM.

                                                                                                daylight in building design 2-11
                             Facades generally have a limited ability to distribute daylight into the depth of a space.
                             Several rules of thumb apply to potential daylighting zones for diffuse skylight and
                             appropriate window design. During the conceptual design phase, the daylighting zone may
                             be considered to be a depth of about two times the window head height [Robbins 1986].

                             Unilateral toplighting can only be used on the top floor of a building. Spaces on lower floors
                             can only be connected to rooflights by core daylighting systems or atria. Rooflights receive
                             light from the brightest regions of the sky, so they are powerful sources of daylight. They
                             do not, however, provide users with a view to the outside, so daylighting strategies that
                             depend exclusively on rooflights are limited to spaces where a view is not necessary.

                             Because toplighting is exposed to high incident sunlight, solar shading is usually essential
                             to prevent overheating. The size of rooflights needs to be carefully balanced to meet
                             lighting, thermal performance, and shading requirements. Various rooflighting shading
                             strategies and systems exist. Rooflights are often glazed with light-diffusing glass to
                             protect the interior from direct sun rays. Light-diffusing glass does not provide solar
                             shading, however, and becomes very bright when hit by direct sunlight, which may cause
                             glare. The use of light shafts to baffle and disperse sunlight is a classical architectural
                             rooflight concept (¡ Gentofte Public Library). The use of awnings is another traditional
                             technique to shade large rooflights (¡ Trapholt Art Museum).

                             Traditionally, toplighting concepts have been used at high latitudes with predominantly
                             cloudy skies, but advanced daylight-redirecting shading systems, such as laser-cut panels,
                             holographic optical elements, and optically treated light shelves, can “cool” rooflights in
                             sunny, hot climates (¡ Center for Desert Architecture, Palm Springs Chamber of Commerce,
                             Park Ridge Primary School).

                             Because the ability of facades to distribute daylight to deep spaces is limited, especially
                             under cloudy skies, bilateral and multilateral lighting is an option for rooms that cannot
                             be lit adequately by only one facade. The design of the building’s fabric determines the
                             availability of daylight on room facades. Atria and courtyards are often used to provide
                             bilateral daylighting (¡ Bertolt Brecht School).

                             Bilateral daylighting with a functional division between facades is a powerful daylighting
                             strategy that can be applied in different ways. One way is to allocate the function of view
                             to the outside to one facade using daylighting systems such as overhangs that shade sunlight
                             but do not obstruct the view. The other facade can be used to distribute daylight to the
                             space (¡ Protestant School). The most common design solution for bilateral daylighting
                             is to combine a window that fulfils the full scope of functions for a large portion of floor
                             space with a clerestory to increase the illuminance level in the depth of the space
                             (¡ OSZ Wirtschaft).

2-12 daylight in buildings
Bilateral sidelighting can provide occupants
with a view to the surrounding landscape.                                                            Figure 2-13:

This strategy allows effective solar shading                                                         Unilateral sidelighting

by moveable systems on a sunny facade                                                                in a laboratory

while the second facade, which is not hit
by direct sunlight, distributes daylight to
the space (¡ Gropius School).

A bilateral combination of sidelighting and
toplighting can distribute daylight to deep
                                                                                                     Figure 2-14:
interior spaces (¡ Gentofte Library). The
                                                                                                     Unilateral rooflight-
primary function of the rooflights is
                                                                                                     ing in an art gallery
usually to distribute daylight while
windows provide occupants with a view
to the outside (¡ Park Ridge Primary
School). Another application of this
strategy is to create areas with their own
specific apertures that provide individual
lighting environments.                                                                               Figure 2-15:

                                                                                                     Bilateral lighting,
Core daylighting systems are optical                                                                 sidelight and
systems in which the daylight-receiving                                                              rooflight
aperture and the light-emitting opening
are far apart. These systems can distribute
daylight to windowless spaces. Occupants
may not notice the difference between
piped daylight and light generated by
artificial light sources. A strategy to make                                                         Figure 2-16:

these systems more cost-effective is to                                                              Sun tracking

use them as distribution systems for                                                                 daylighting system

artificial light as well. Core daylighting                                                           with fresnell

systems are usually designed to pipe direct                                                          collector and liquid

sunlight. There are various types of core                                                            light pipe

daylighting systems — sun-tracking as well
as fixed systems that use optical fibres or
light ducts.

2.2.5. Relation to Adjacent Spaces
Clerestories in corridor walls of offices can distribute daylight to these otherwise windowless
spaces; for shallow offices, this is a suitable strategy (¡ DIN-Building). Daylight-redirecting
systems can contribute to the distribution of daylight to these spaces in the core of a
building; daylight-redirecting systems should be applied simultaneously to control glare.
Windows facing an atrium have less daylight potential than windows facing an open

                                                                                                  daylight in building design 2-13
                                 courtyard because the glazed roof reduces the luminous flux. High reflectances within an
                                 atrium space can increase the depth of light penetration (¡ Dragvoll University Center).

                                 Even if borrowed light from interior windows does not significantly increase daylight levels
                                 to an interior space, the presence of these windows may improve light distribution and
                                 make a visual link between a space located in the core of a building and other zones of
                                 the building. The view to a daylit environment or even to an interior building landscape
                                 can increase the attractiveness of otherwise windowless spaces.

                                 2.2.6. Finishing, Furnishing, and Using a Space
                                 Interior finishing has to be part of the daylighting strategy. Daylight-redirecting strategies
                                 usually direct daylight to the ceiling of a room. The reflectance characteristics of the ceiling
                                 therefore influence the way daylight will be distributed. Specular in-plane ceiling surfaces
                                 reflect redirected light deep into the space but may be a source of glare. Specular out-of-
                                 plane ceiling surfaces can be shaped to deflect redirected daylight to specific areas in the
                                 room (¡ Geyssel Office Building). These surfaces can act as reflectors for artificial light
                                 as well. A diffuse ceiling of high reflectance can also distribute light from daylight-
                                 redirecting systems, which may be more comfortable for occupants than a highly reflecting
                                 environment. The reflectance of walls, floor, and furniture also have a large influence on
                                 the impression created by a space. The floor reflectance should not be too low (>0.3).

                                 Designers often assume that lighting requirements are homogeneous throughout a space
                                 and thus aim to provide uniform lighting levels, but surveys in occupied rooms show that
                                 there are patterns in how spaces are used. For example, in a cellular office occupied by
                                 one person, the desk is usually placed in the window area.

                                 The furnishing of a space represents a frozen image of activities in the space. It affects where
                                 occupants do certain tasks. Thus, furnishing acts as a specification of lighting requirements.
                                 If the real use of a space can be determined, designs should be based on this information
                                 rather than on the assumption that a uniform luminous environment is required.

                             2.3. Design Strategies for Daylighting Systems

                                 As outlined above, the application of daylighting systems is only one constituent of a
                                 daylighting strategy. Although a poor selection of systems can spoil the performance of a
                                 building with good daylight potential, a sound selection cannot compensate for errors and
                                 omissions in previous design stages.

2-14 daylight in buildings
To select a system, the designer must understand:
    •    the function of the window or other opening(s),
    •    the function of the system, and
    •    the interplay of the system with other systems.

A reasonable selection of systems should reduce the negative effects of windows and
enhance daylight performance without interfering with other desirable effects of windows
for all design cases (all seasons and sky conditions).

Daylighting systems can be categorised by many characteristics. When selecting a system,
the designer must be aware of all of its properties. Function and performance parameters
have the most pronounced effect on performance, but costs and details related to the skin
of the building are also important. As for many decision within the design process there
exists no definite procedure how to select a daylighting system. The ultimate criterion is
the performance of the overall design solution.

                                                                                                    Figure 2-17:

                                                                                                    Functions and design

                                                                                                    considerations of

                                                                                                    windows and daylight-

                                                                                                    ing systems

Windows and rooflights have different roles in a daylighting strategy. The ambience of
spaces receiving skylight is completely different from that of spaces receiving sidelight. For
example, the design of Le Corbusier’s “Le couvent de la Tourette” emphasises the different
nature of skylight and sidelight. In this design, skylight is used only in spaces that play a
significant role in religious life; all secular spaces receive sidelight.

                                                                                                 daylight in building design 2-15
                             Rooflights are usually not designed for a view to the outside; therefore, obstructing
                             elements such as deep light shafts or non-transparent daylight systems can be applied in
                             rooflight design. The control of glare with such systems is much easier than with sidelighting
                             designs, which must provide occupants with a view to the outside. Solar shading is a crucial
                             issue with rooflighting. One design strategy for rooflighting in sunny hot climates is to use
                             a very small aperture and to apply innovative daylighting systems to distribute the light
                             homogeneously in the space (¡ Waterford School, International Centre for Desert
                             Architecture). In classrooms of the Park Ridge Primary School in the sunny but temperate
                             climate of Melbourne in southern Australia, tunnel lights are used to exclude direct
                             sunlight and to distribute skylight to the space (¡ Park Ridge Primary School).

                             Shading systems for rooflights, such as sun-protecting mirror elements, prismatic panels
                             (Chapter 4.5), and directional selective shading systems using holographic optical elements
                             (Chapter 4.11) can be applied to large glazed roof areas in higher latitudes. When situated
                             in the window pane, these systems are protected from dust and require little maintenance.
                             These systems need to be adjusted to the individual application.

                             2.3.1. Function of Systems

                             The system matrix for the division and description of daylighting systems that is included
                             in Chapter 4.2 of this book makes a distinction between two major categories
                             of daylighting systems: those with and those without shading. This division is useful
                             for building designers. Daylight-redirecting systems that do not shade usually need to be
                             complemented by other window systems; shading systems might be applied as stand-alone
                             systems for windows or window areas.

                             Daylighting systems have three major functions:
                                 •    solar shading,
                                 •    protection from glare,
                                 •    redirection of daylight.

2-16 daylight in buildings
Windows need protection from glare and solar shading in order to create acceptable interior
conditions. The redirection of daylight can save energy but is not an indispensable
function. The view to the outside is not a function of a daylighting system but a primary
function of the window itself; the impact of daylighting systems on the view to the
outside needs to be considered carefully.

Some systems, such as exterior louvered blinds                                                     Figure 2-18:
(Chapter 4.4), are designed to satisfy all                                                         Exterior glazed
functions of a standard window as a stand-                                                         lamellas for
alone system. But, as outlined above, such a                                                       solar shading
“one size fits all” system, which usually covers
the whole window area, will, in most cases,
have a poor performance. A good selection of
systems means a good mixture of systems.

Reflecting profiles in the cavity of the glass are
designed for solar shading, redirection of daylight,                                               Figure 2-19:

and glare control, but create considerable                                                         Light guiding

obstructions to view. When this technology is                                                      shade for solar

applied to a facade, low incident daylight is                                                      shading and

redirected to the ceiling, and high incident                                                       redirection of

daylight is rejected. This strategy performs well                                                  day-light

under sunny skies in high latitudes when oriented
to the south (in the northern hemisphere), but it
has poor daylighting performance under overcast conditions. It is primarily a selective
shading strategy that can be applied to control the thermal performance of large areas of

Architectural design features often to some extent fulfil or
support the functions of daylighting systems, but they cannot                                      Figure 2-20:

address the full range of exterior conditions, so additional                                       Interior fabric to

daylighting systems are generally needed. An overhang, for                                         protect from glare

example, acts as a solar shading system but only for high sun
positions. It does not protect from glare or redirect light into the
space. Fins act to some extent as solar shading devices,
attenuating and redirecting sunlight and partially controlling
glare, but they are not a stand-alone daylighting system. These
architectural design features selectively attenuate daylight so that
simple daylighting systems can be added to supply missing
functions. Other examples of architectural features that shape daylight are arcades, atria,
balconies, and deep window reveals. The performance of these elements can only be
evaluated within the context of a specific design solution, so surveys and case studies are
useful assessment resources. In addition to the survey of architectural solutions included in

                                                                                                daylight in building design 2-17
                             this source book, some very useful compilations of case studies have been published in recent
                             years [Fontoynont 1999, LUMEN 1995, IEA SHC Task 21 Daylight in buildings: 15 Case Studies
                             from Around the World].

                             In high latitudes with predominantly cloudy skies where the exterior illuminance on
                             winter days at noon is often even less than 5,000 lux, measures to increase daylight
                             during winter are appropriate. A high window with a sloping lintel has proven to be more
                             efficient in this case than most daylight-redirecting systems. The application of glass with
                             high light transmission is also very useful. Because a large window designed for low levels
                             of daylight on overcast days is vulnerable to overheating and glare on bright days, effective
                             shading systems are needed to make this daylighting strategy work.

                             A light shelf (Chapter 4.3) combines solar shading and sunlight redirection, improving the
                             distribution of daylight and allowing a view through the lower part of the window. Light
                             shelves are applicable in sunny climates in mid-latitudes for south orientations (in the
                             northern hemisphere). Light shelves are a classical device in the daylighting toolbox.

                             Other systems are designed for only one function; zenithal light-guiding glass (Chapter 4.10),
                             for example, redirects sunlight but does not provide solar shading or glare protection.
                             Interior roller blinds primarily protect against glare, but they only have a limited effect on
                             solar shading and usually do not redirect daylight. A pivoting window adapts to summer
                             and winter light availability conditions but has no effect on daylight distribution.

                             2.3.2. Location

                             The location of a daylighting system can be described in relation to the window pane as
                             exterior, interior, or within the pane. Some complex systems such as the anidolic ceiling
                             (Chapter 4.12) combine exterior and interior elements. The location of a daylighting
                             system can affect the thermal performance of the building.

                             Exterior systems are most suitable for solar shading; interior systems allow for solar gains.
                             Systems located in the cavity of the glass or within a double facade can be applied as part
                             of an advanced ventilation strategy to serve as exhausts in summer and solar collectors
                             in winter.

2-18 daylight in buildings
Exterior devices are costly because they have to be constructed to resist all weather
conditions. Moveable exterior systems require a lot of maintenance and often collect dust.
Interior systems are much less expensive, but they have only a limited solar
shading effect.

The designer should therefore aim to find the right size and position of windows and use
fixed elements in the window design if applicable, so the need for moveable exterior
systems can be reduced. Innovative systems are often located in the window pane. They
control daylight as a function of incidence angle but affect the view to the outside.

2.3.3. Ability to Change

Because one of the main functions of daylighting systems is to adapt the building to
changing sky conditions, the ability to change is an important characteristic of these
systems. A system itself does not necessarily need to change. A design using fixed systems
that reflect the trajectory of the sun can be sensitive to sky conditions, for example.
Orientations to the south (in the northern hemisphere) are especially appropriate for such
a design. Although fixed systems, such as overhangs, sun shades, horizontal lamellas, and
fins, are useful for solar shading, they do not control glare; therefore, another system that
controls glare needs to be added to make these design solutions work. Because the glare
protection device is not used for solar shading in this case, an interior system can be applied
(¡ OSZ Wirtschaft).

Many buildings in hot climates have in recent years been designed for solar shading rather
than for daylighting. Reducing cooling loads was the driving force in these designs. Sun-
shading glass has been used to exclude solar radiation, and window function has been
limited to providing occupants with a view to the outside. Today, advanced daylighting
systems in combination with advanced controls can bring daylight deep into a space and
reduce cooling loads relative to those experienced with artificial lighting. If thermal loads
are a major concern, tracking systems can be used to regulate daylight levels.

                                                                                                  daylight in building design 2-19
                             2.3.4. Transparency
                             Because a primary function of windows is to provide occupants with a view to the
                             outside, the transparency of daylighting systems is a major issue. The construction material
                             of a daylighting system need not necessarily be transparent itself in order to provide a view
                             out; the subjective impression of visual contact to the outside is most important. The
                             function of a system to protect from glare inevitably affects the view to the outside. Sun
                             shading and the redirection of daylight affect the view as well.

                             Some advanced systems, such as holographic optical elements, laser-cut panels, and light
                             shelves, aim to shade or redirect daylight from some incidence angles while not interfering
                             to any great extent with the view to the outside (see Chapter 4). These systems do not
                             control glare. Fixed daylighting systems that do control glare, such as sun-protecting mirror
                             elements in the cavity of the glass, anidolic ceilings, and light-guiding shades, do not
                             provide occupants with a view to the outside.

                             Louvers and blinds and other moveable systems that can be recessed are designed to shade
                             and protect from glare when needed, but they do not interfere with view when they are
                             recessed. The transparency of these systems depends on the operating conditions.

                             Electrochromic glass can adjust the transmission of radiation over a wide range without
                             changing the distribution of daylight. Glass with light transmission that varies depending
                             on the amount of incident daylight or the temperature is a promising technology that has
                             been developed in laboratories.

2-20 daylight in buildings

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